It might be cryptopunk, but it’s absolutely not useful.
As has been pointed out, on flash drives you can’t overwrite a certain sector, as the flash device opaquely maps it to any sector it wants. Not even filling all sectors is a guarantee, as the disk has some hidden spare sectors, and the confidential data could end up there, if the disk decides to swap some sectors with spare sectors.
Oh, and of course… if you have more than one partition (main, boot, swap…) on one SSD, then all of them would have to be filled simultaneously, to reduce the risk of some sectors not being overwritten.
No, because the information that the sector is unused is not stored in that sector, but somewhere else.
So yes. Encrypted disk is a much better choice.
You could even create an encrypted mini-disk that can be deleted after use. (Just create a file of the desired size, and then encrypt/format it like any disk)
RAM cleaning is the closest to useful, but of course wipes the disk cache, too. Can also just reboot 